1. Field of the Invention
This invention relates to a coating composition for coating the inner wall of a Braun tube (cathode-ray tube). More particularly, the invention relates to a coating composition containing electroconductive graphite which is used for coating the inner wall surface of the funnel glass of a cathode-ray tube.
2. Description of Prior Art
The inner wall surface of a funnel glass of a cathode-ray tube is provided with an electroconductive coating. This electroconductive coating film plays an important part in functions to accelerate electron beams by applying a high voltage and to capture secondary electrons which are emitted from a shadow mask, magnetic shielding materials and a fluorescent screen.
The electroconductive coating of this kind is formed by spraying or brushing a coating composition to the inner wall surface of a funnel, which composition contains fine particles of electroconductive substance. This coating procedure is followed by a drying step and a baking step in the air.
The cathode-ray tube is produced by a process such that a funnel section the inside of which is provided with an electroconductive coating composition and a separately made fluorescent screen section are bonded together with a low-melting glass at about 440.degree. C. to prepare a tubular body. An electron gun is then built into the tubular body and the inside of the tube is evacuated by heating and exhausting. Because the coating film formed on the inner wall of the funnel before the evacuation adsorbs moisture, carbon dioxide and other gases from the surrounding air, the adsorbed gases must be removed by heating treatment and exhausting under a reduced pressure just before the process of sealing up of the cathode-ray tube.
Even when the above evacuation is successfully carried out, trace quantities of adsorbed gases remain in the cathode-ray tube and, during the operation of the cathode-ray tube, the adsorbed gases are slowly released. The released gases react with the cathode and the function of the cathode is deteriorated, and ultimately, the emission of electrons is damaged. For this reason, it is eagerly demanded that the quantities of gases released after the sealing of the cathode-ray tube are reduced so as to prolong the service life of the cathode-ray tube.
In the above-mentioned process for producing cathode-ray tubes, if the electroconductive coating peels off from the inner wall of the funnel, arc discharge and electrical leakage are caused to occur during the working of the cathode-ray tube. Arc discharge and electrical leakage impair the high voltage stability of the tube, so that the electroconductive coating must be tightly bonded to the inner wall of funnel so as to prevent the coating from peeling off even when it is subjected to vibration or other shocks. In addition, it is necessary to regulate the electrical resistance of the coating film into a certain range in order to reduce the spark currents.
For the above reason, it is necessary that the coating composition of this kind can be applied without difficulty and it is formed into a smooth and uniform coating film without causing any cracking or wrinkling. Furthermore, it is required to minimize the dripping of the coating composition. Still further, the degassing of formed graphite layer must be effective and, after the degassing, the graphite layer should not release any gas in the condition of vacuum.
The coating composition according to the present invention is made by dispersing fine particles of graphite as an electroconductive substance in an aqueous medium which contains a dispersing agent and potassium silicate as an adhesive. If necessary, in order to regulate the electrical resistance of the coating film, fine particles of metal oxides or metal carbides such as iron oxide, titanium oxide and silicon carbide, can be additionally dispersed.
It is possible to use only graphite particles as an electroconductive material; However because the spark current is relatively large in this case, fine particles of graphite and metal oxide are commonly used together. That is, the graphite gives electroconductivity to lower the electrical resistance of a coating film, while, the metal oxide functions as a filler and at the same time, it functions to raise the electrical resistance of coating films, like the silicates adhesives. Therefore, the electrical resistance and adhesive strength of a coating film can be adjusted to certain values by changing the compounding ratios of these materials.
Exemplified as the metal oxides are the oxides of Fe, Ti, Co, Ni, Cr, Mn, Al and Si, as disclosed in, for example, Japanese Patent Publication No. Sho 55-2042, Japanese Patent Publication No. Hei 3-59542 and Japanese Patent Publication No. Sho 63-45428. Coating compositions containing oxides of iron or titanium are commercially available. It is known as disclosed in the above-mentioned Japanese Patent Publication No. Sho 63-45428 that, in order to disperse stably both negatively charged particles and positively charged particles in a negatively charged dispersion medium, negatively charged graphite particles and positively charged TiO.sub.2 particles are agglomerated together and negatively charged SiO.sub.2 particles are stuck around the agglomerated particles to obtain compound particles to be dispersed. It is also known as disclosed in Japanese Patent Publication No. Sho 61-20990 that silicon carbide particles in addition to graphite particles are added in order to prevent a coating film from peeling by improving its adhesive property.
The particle diameters of the above-mentioned metallic compounds including metal oxides and metal carbides are in the range of about 0.1 to 1 .mu.m. As iron oxide, .alpha.-Fe.sub.2 O.sub.3 is used and as titanium oxide, rutile type one is used.
The graphite as an electroconductive material has a particle size distribution in the range of about 0.5 to 10 .mu.m. In practice, both natural graphite and artificial graphite can be used.
As disclosed in Japanese Laid-Open Patent Publication No. Sho 52-52362 and Japanese Patent Publication No. Sho 63-45428, adhesives are exemplified by lithium silicate, potassium silicate and sodium silicate. Among them, potassium silicate is widely used in industrial practice. This is due to the fact that the coating films using lithium silicate are liable to be peeled off from the glass surface of cathode-ray tube although its moisture adsorbing property is low and, in the case of sodium silicate, moisture adsorption is intense and the formed coating is soft.
The molar ratio of silicon dioxide and potassium oxide (SiO.sub.2 /K.sub.2 O) in the potassium silicate was about 2.8 to 3.8 in the conventional art as disclosed in e.g., Japanese Patent Publication No. Sho 55-2042.
As the above-mentioned dispersing agent, carboxymethyl cellulose or the like is used.
The compositions of the coating materials are disclosed in the above-mentioned patent gazettes. The compounding ratio of graphite particles to potassium silicate is generally determined in accordance with a desired value in electrical resistance. This can be varied diversely according to the configuration and specification of the cathode-ray tubes to be produced. When the quantity of graphite is increased, the electrical resistance of a coating film is naturally lowered and the adhesive strength of coating film to the inner wall of a funnel is lowered. On the other hand, when the quantity of potassium silicate is increased, the electrical resistance is increased and the adhesive strength is also improved; however, the undesirable phenomena of blistering and gas generation are caused to occur in the coating film.
In the case of a coating composition in which the dispersion particles are only graphite, it is advisable to use about 2/3 of graphite and the remainder 1/3 of potassium silicate.
In the case of an inner coating composition having the so-called soft-flash effect, the quantities of graphite, metal oxide and potassium silicate are about 1/3, respectively.
The quantity of dispersing agent is about 0.1 to 3% by weight. The dispersing agent has the effect to prevent the graphite particles and metal oxide particles from precipitation to maintain them in a stable suspended state, however, the peeling of coating film is liable to occur when the dispersing agent is added to excess.
The quantity of water in the coating composition is not constant because it is varied according to the manner of applying (spray-coating, brush-coating, etc.), the desired thickness of coating film and required workability. It is generally determined in the range of about 60 to 80% by weight.